The present invention relates to an illumination system, in particular for endoscopy, and more particularly a full spectrum illumination system using light-emitting diodes (LED) and/or semiconductor lasers.
Illumination systems for endoscopy, microscopy and similar optical imaging applications have for many years utilized arc lamp or halogen technology as the light source of choice. More recently, various forms of solid state light sources such as light emitting diodes or diode lasers have been introduced for use in some of these imaging applications. Due to the output brightness or output spectrum limitations of these solid state light sources, the use of LEDs and/or laser diodes has, until recently, been limited to optical imaging applications where low light levels are sufficient or where narrow spectrum illumination is required/desired.
Achieving sufficiently bright, full visible spectrum illumination with solid state light sources has remained challenging for a number of reasons.
a) Firstly, LED technology has been improving, but started far behind that of lamp technology in terms of total light output. Increasingly higher light outputs are now available, but light from a single phosphor-coated (“white”) LED, for example, is still orders of magnitude below that of an arc lamp.
b) Alternatively light from multiple, different colored (e.g. red, green and blue) LEDs can be combined using dichroic mirrors to “source” emitting over a wide spectral range. The imaging applications mentioned above, however, generally require coupling light into liquid, fiberoptic, or rod lens light guides. Such optical light guides typically have both a small physical aperture with dimensions of a few mm across and a constrained/limited numerical aperture (NA). Moreover, etendue considerations rapidly constrain the practical implementation of such combined source illumination systems.
c) Should the etendue considerations with a multiple different colored LED arrangement be overcome by a suitable arrangement of sources and dichroics with optical path lengths that are carefully equalized, then other implementation issues arise with respect to effective cooling and cost.
Finally, although output brightness of red and blue LEDs has reached levels at which they can produce light with a brightness substantially equivalent to that of the red and blue portions of an arc lamp or a halogen lamp spectrum, the output of green LEDs tends to be substantially less than the green light produced by lamps.
It would therefore be desirable and advantageous to address this problem and to obviate other prior art shortcomings by providing a cost-effective and reliable illuminator utilizing solid state light sources to produce a bright, color balanced, broad spectrum visible light output that may be effectively coupled to an optical light guide. It would also be desirable to include in such illuminator and in the resulting light emission, other light sources for UV or NIR illumination (e.g. for fluorescence excitation of tissue).